Low oxygen pressures have been shown to activate regions of the brain which exert facilitatory and inhibitory influences on the centers concerned with the control of breathing, and thus the net response to signals on those centers may be amplified or modulated. The proposed experiments now incorporate CO2 into that scheme and hypothesize that low CO2 may enhance general 'arousal' characteristics and thus, the system controlling breathing. To explore the possibility a number of experiments on the control of breathing, all associated with low CO2 in the alveolar gas, are proposed. Another set of experiments are designed to test the hypothesis that controller gain of the system concerned with the chemical control of breathing is set to be proportional to rate of oxygen uptake, no matter whether metabolic rate changes in response to activity, or whether it varies intraspecifically as a function of body size.